My turn to chime in here. When building a gaming system, work backwards. Pick your monitor size/resolution first, then pick parts to drive them at the FPS you want.

If you're using anything less than a 24" or 1920x1080, the GTX 280 is overkill, IMO. Also, I've heard of TONS of problems with people using Nvidia chipsets having their mobo's blow up, far more than the Intel chipsets. If you're set on SLI, go for it, but usually you don't get twice the framerate by dropping in a second card at a later date. You're better off buying the newest generation single card solution.

CPU - Get the Q9550, you're spending an extra $200 for 0.17Ghz. I can almost guarantee you could mildly overclock it to 3.0 without a voltage change just by bumping the FSB to 352 from 333. Stick with the Quad if you're doing anything else besides gaming and surfing on it, but if it's a pure gamer, the Duo's are faster. Very few games utilize more than 2 cores as yet, and benefit more from the higher clock speeds on the Duo.

Case - The Stackers and Black Pearls are awesome cases. If they're in your budget you'd be much happier with them in the long run.